This invention relates to a pneumatic radial tire, for example, a heavy duty pneumatic radial tire having a particular belt construction. More particularly it relates to a tire improved in durability by reducing interlaminar shearing strains at ends of narrow belts among laminated belts whose cords intersect between the belts.
A construction of a conventional pneumatic radial tire is illustrated by way of example in FIG. 1 which is a sectional view of a right half of the tire taken along a plane including a rotating axis of the tire. As shown in FIG. 1, the tire comprises a pair of beads 13, a carcass 12 radially extending between the beads and anchored by the beads, a tread 14 located outward of the carcass and adapted to be in contact with a load, belt layers extending between the carcass 12 and the tread 14, and the like. As the invention mainly relates to belt layers, belt layers will be explained in more detail in the specification. A usual heavy duty pneumatic radial tire is constructed by laminating more than two belts as shown in FIG. 1. In order to make it easy to understand the invention, it will be explained referring to laminated belts made of two belts.
A construction of hitherto used belt layers is shown as typical example in FIG. 2a. In the drawing, reference numeral 1 denotes a laminated belt comprising a first belt layer 1 having a belt width 2W2, and a second belt layer 1 adjacent outward of the first belt layer and having a belt width 2WB narrower than the width 2WA. Cords of the first belt layer 1 are inclined in a manner with right ends being raised as viewed from a tread of the tire at angles 25.degree. with circumferential directions. It is indicated in the drawings as "R 25.degree. ". On the other hand, cords of the second belt layer 1B are inclined in a manner with left ends being raised as viewed from the tread at angels 25.degree. with the circumferential directions. It is indicated in the drawings as "L 25.degree. ". The cords of the first and second belt layers intersect with each other to form main reinforcing belts. The "main reinforcing belt" used herein is intended to mean "two adjacent laminated belt layers whose cords have the maximum strength among cords of all belt layers and the cords of the two belt layers intersect with each other with respect to circumferential directions". Widths of the first and second belt layers 1A and 1B are somewhat different and these belt layers are laminated with their cords intersecting with each other with respect to the circumferential directions, thereby maintaining the rigidity of the laminated belts to restrain circumferential elongations and diameter enlargement of the tire or to exhibit so-called hoop effect".
When a pneumatic radial tire is running, interlaminar shearing strains .gamma. occur at 2 caused by running in addition, to interlaminar shearing strains owing to the inflation air pressure. The interlaminar shearing strains indicated by ".gamma." are limited to the interlaminar shearing strains caused when the tire is running except the interlaminar shearing stains due to the filled air pressure. As shown in FIG. 2b, the interlaminar shearing strains .gamma. are distributed in a manner that the maximum value .gamma.1 are is at a belt end 1BA of the second belt layer 1B having the narrower width and the strains are progressively decreased to the center c. With such a pneumatic radial tire, cracks 3 often occur at the belt ends 1Ba of the second belt layer 1B and grow between the belt layers at 2 into a serious fault.
In order to reduce the interlaminar shearing strains .gamma., arrangements of the belt layers have been proposed as shown in FIGS. 3a and 3b wherein like components are designated by the same reference numerals as those in FIG. 2a. In FIG. 3a, a width 2WB2 of a second belt layer 1B is narrower than the width 2B1 of the belt shown in FIG. 2a. In this case, the maximum interlaminar shearing strains .gamma.2 of the second belt layer 1B are considerably decreased in comparison with the value .gamma.1 shown in FIG. 2a (refer to broken lines in FIG. 3b). In FIG. 4a, angles .theta. of cords of all or part (adjacent two layers in this case) of the belt layers of the laminated belt layer 1 are relative large angles 30.degree.. Interlaminar shearing strains .gamma.3 between the belt layers at 2 are reduced in comparison with the strains .gamma.1 in FIG. 2b whose cord angles are 25.degree. (refer to broken lines in FIG. 4b). However, belt rigidities of the overall laminated belt 1 of the tires shown in FIGS. 3a and 4a are considerably reduced and the diameter enlargement is increased so that durability of the tires is lowered.
In more detail, with a tire whose rigidity of belt is lowered, as shown in FIG. 5, when the tire is filled with the normal inner pressure, an outer diameter elongation becomes larger to a point Q2 in comparison with the diameter enlargement Q1 of a tire whose rigidity of belt is high, so that strains of inner rubber of the tire become larger. Under such a condition, upon running of the tire the diameter enlargement is further developed as shown by a line rising from the points Q1' and Q2' to the right in FIG. 5. In general, the larger the diameter enlargement before use, the larger are the strains in the inner rubber of the tire caused by the diameter enlargement in running. Such large strains accelerate the deterioration of the rubber to lower the durability of the tire.